This invention relates to a tone signal generation device of a waveshape memory reading type and, more particularly, to a tone signal generation device in which a waveshape is selectively read out from among a plurality of different waveshapes stored in a waveshape memory. The invention further relates to a tone signal generation device in which a waveshape memory stores plural waveshapes corresponding to partial time sections in an entire tone generation duration from the start of sounding of a tone to the end thereof.
U.S. Pat. No. 4,383,462 discloses an electronic musical instrument in which an entire waveshape from the start of sounding of a tone to the end thereof or partial waveshapes of plural periods are stored in a waveshape memory and a tone signal of a high quality closely resembling a tone of a natural musical instrument is produced by accessing this waveshape memory. For enabling selection of several kinds of tone colors in such electronic musical instrument, waveshapes corresponding to the respective tone colors (an entire waveshape or partial waveshapes of plural periods) must be respectively stored in a waveshape memory. Besides, for affording variety to the tone color depending upon a key touch or a tone pitch of a tone to be generated, waveshapes corresponding to several levels of the key touch strength or waveshapes corresponding to several tone pitches or tone ranges must be respectively stored in the waveshape memory.
In storing different entire waveshapes of several kinds in a waveshape memory, the prior art device employs a common coding form without considering individual characteristics of original waveshapes. This results in undue increase in the memory capacity or, if the memory capacity is held below a certain limit, resolution of sampled waveshape is deteriorated. If, for example, such a coding form is adopted as a pulse code modulation system (hereinafter called "PCM system") which is capable of reproducing with a high resolution a waveshape changing in a complicated manner, a large memory capacity is required and this large memory capacity is wasted when the stored waveshape is a relatively simple waveshape which does not require a high resolution and, accordingly, the memory capacity increases more than necessary as a whole. If, conversely, saving of the memory capacity is intended by adopting such a coding form as a differential pulse code modulation system (hereinafter called "DPCM" system) suitable for saving the memory capacity, resolution of the reproduced waveshape is deteriorated with respect to a waveshape which undergoes a complicated change, though this construction will be suitable for a waveshape of a relatively simple change.
Also in storing a series of partial waveshapes of plural periods in a waveshape memory, the coding form of waveshape data to be stored in the waveshape memory has conventionally been the same for all partial waveshapes and the PCM system, is generally employed. The PCM system is suitable for accurately reproducing a waveshape changing timewise in a complicate manner with a high resolution but requires a relatively large number of bits for waveshape data of one sample point with a result that the memory capacity tends to become large. This tendency is strong particularly when waveshapes of plural periods are stored as described above, resulting in provision of a waveshape memory of a very large memory capacity. On the other hand, the employment of a coding form (e.g. the DPCM system) capable of saving the memory capacity causes the problem that the accuracy in reproduction of a waveshape is sacrificed in a portion in which the waveshape changes in a relatively complicate manner.